Mechanical changes in living tissue correlate with pathological changes. As between healthy and pathological tissue, the shear elastic modulus (stiffness) and viscosity can vary significantly. With the advent of ultrasound elasticity imaging development over the past decade, many clinical studies have shown that tissue visco-elastic properties provide useful information to physicians for better cancer diagnosis and therapy assessment.
One method for measuring tissue mechanical properties is ultrasound shear wave elastography. It utilizes acoustic radiation force (ARF) to generate shear waves in soft tissue and subsequently tracks shear wave displacement to estimate tissue elasticity and viscosity. An application of this technique is the non-invasive measurement of liver stiffness to stage liver fibrosis and cirrhosis.
Interrogation by ultrasound, for purposes of medical imaging, often makes use of longitudinal waves. In body tissue, the ultrasound propagates in wave form. In effect, particles all along the propagation path vibrate, in place, back and forth, the vibration occurring in the direction of propagation. The vibrations create compressions and rarefactions. These are modeled as the peaks and valleys of a sinusoid. Energy is conveyed to the target and back by means of the oscillatory particle movements.
An ultrasound shear (or transverse) wave, by contrast, is characterized by back and forth in-place movement that is perpendicular to the direction of propagation. Oscillation one way creates the peaks, and the other way creates the valleys. The wave is comprised of components, each oscillating at its own frequency. It is the propagation speed of the wave envelope, or “group velocity”, which is sought.
First, a focused longitudinal-wave push pulse is issued. It is a high intensity, long duration and narrow bandwidth signal. The push pulse creates a shear wave. The focal depth has been selected, at the outset, so that the shear wave travels through a region of interest (ROI). Push pulses can be fired repeatedly for multiple measurements to increase accuracy of the estimation of shear wave velocity. A typical repetition rate is 100 Hz.
A longitudinal-wave tracking pulse is issued to the ROI to assess, at the sampling point (or “lateral location”), the amplitude of the shear wave over a certain observation period (on the order of 10 ms). The measurement is of the body-tissue displacement perpendicular to the lateral direction of shear-wave propagation, that propagation being away from the region of excitation (ROE) at the push focus. The time period between the peak displacement and the push pulse responsible is called the time-to-peak (TTP). An example of the TTP technique is found in U.S. Patent Publication No. 2008/0249408 to Palmeri et al. (hereinafter “Palmeri”), the disclosure of which is incorporated herein by reference in its entirety.
The TTP is similarly determined for a number of lateral locations located, in mutual linear alignment along the propagation path, outward from the ROE. Time delays between peaks of different locations can be derived, as in acoustic radiation force impulse (ARFI) imaging.
Based on known distances between the lateral locations, a functional relationship can be estimated between distance the shear wave propagates and the time over which the distance is traversed.
From location to location, as the shear wave propagates outward from the ROE, geometrical spreading and viscosity cause the displacements to decay. Therefore, simply detecting, location by location, when a fixed displacement is achieved, is not a feasible means for determining group velocity.
In view of this, TTP assumes that a wave arrives at, or passes over, a lateral location when the peak displacement of the wave occurs at that location. It further assumes that the peak displacement outside the ROE travels at the group velocity of the shear wave.
Palmeri finds that these assumptions hold for purely elastic or mildly dispersive media.